The present invention relates in general to high voltage generator circuitry, and in particular to various circuit techniques for controlling the ramp rate and amplitude of the output voltage of a charge pump circuit.
In certain types of semiconductor devices such as non-volatile memories, voltage levels higher than the power supply voltage are required. For example, in electrically erasable programmable read only memories (EEPROMs), memory cells are programmed or erased by a mechanism known as Fowler-Nordheim tunneling (FNT). FNT occurs when high enough voltages are applied across the tunnel oxide of the memory cell to generate the necessary electric field (E-field) to induce charge to cross the tunnel. Charge pump techniques are typically used to internally generate the higher programming voltage (VPP) from the regular power supply voltage.
Conventional charge pump circuits generate VPP at a relatively fast rate, resulting in larger E-fields across the tunnel oxide. The tunnel oxide, which is typically made up of a thin layer of silicon dioxide, tends to degrade over time as it undergoes repeated electron tunneling. The degradation is cumulative and is generally proportional to the E-field experienced by the oxide at each tunneling event. A charge pump circuit with fast VPP ramp rate, therefore, subjects the tunnel oxide to larger E-fields for a longer period throughout the duration of each tunneling event. The resulting damage to the tunnel oxide over time causes reliability problems and shortens the life of the product. This problem is exacerbated as processing dimensions are shrunk and tunnels oxides are made thinner.
There is therefore a need for high voltage generator circuits with well controlled ramp rates.